Session A7
Paper #56
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NUCLEAR FUSION THROUGH TOKAMAK CONFINEMENT SYSTEMS
John Hiller, [email protected], Lora 3:00, Benjamin Page, [email protected], Mahboobin 10:00
Abstract—Nuclear Fusion is an experimental form of energy
which involves the heating and confinement of different
hydrogen isotopes until they can fuse together and produce
energy. Tokamaks are a form of magnetic confinement system
originally developed in 1951 by soviet physicists Andrei
Sakharov and Igor Tamm. Tokamaks make use of a toroidal,
or donut shape, configuration which together with an induced
electric field confines the hydrogen plasma used to fuel
nuclear fusion into a closed loop. Within a tokamak, the
plasma fuel acts as the secondary winding of a transformer,
while the primary winding is an external coil. The external
coil’s winding induces a current in the plasma and coupled
with field coils surrounding the toroidal shape and generates
the magnetic field. Tokamaks offer a design for devices which
can facilitate nuclear fusion that has the potential for
international implementation and that has received a great
amount of research. Currently numerous tokamaks have been
or are being developed for research purposes, including JET
and ITER. Research tokamaks serve as both investigative tools
and examples of the potential of fusion power. Although the
implementation of tokamaks offering energy on a large enough
scale for commercial use is still decades away, they still
represent a promising option for the future of nuclear fusion.
and fuse the ions together [1]. Thus, the ions fuse and cause a
release of energy.
Due to the great amount of invested research, the
behavior of fusion reactions is well understood and represent a
future of harnessable clean energy. As of today, the main
challenges remaining with fusion power lie more in the design
and construction of commercial fusion reactors, as the
conditions required for a fusion reaction are already
understood. Although small-scale research reactors, which are
present internationally, offer a valuable source of information
for the behavior of aspects of fusion reactions, such as the
plasma behavior devices, and what will be required for their
operation, they are not able to supply energy at levels high
enough to be used for any sort of commercial implementation.
Fusion vs Fission
In a fission reaction, the nucleus of a uranium atom
absorbs a neutron causing it to become unstable and break up.
This results in the production of additional neutrons which hit
more uranium atoms and trigger a chain reaction [2]. Although
fusion reactions are much harder to facilitate, they do not
involve any form of chain reaction like that which is present in
fission. As stated, fusion reactions simply require its plasma
fuel to be heated and contained. This means there is no risk of
a runaway process in a fusion reaction, making fusion an allaround safer method of harnessing nuclear power.
Key Words—ITER, JET, K-DEMO, K-STAR, Magnetic
Confinement, Nuclear Fusion, Tokamaks
AN INTRODUCTION TO NUCLEAR FUSION
The Fuel for Fusion
Fusion is the process in which hydrogen atoms fuse
together forming helium, and converting matter into energy.
When heated to high enough temperatures hydrogen gas is
converted into a plasma causing its negatively charged
electrons and positively charged atomic nuclei, or ions, to
become separate [1]. On earth fusion is impossible under
normal circumstances. This is because the repulsive
electrostatic forces between ions of separate atoms of
hydrogen prevent any collision from occurring. However, if
hydrogen atoms experience a high enough temperature
increase fusion can become possible. The temperature increase
causes the speed of individual ions to become so great that they
can overcome the repulsive electrostatic force and move near
enough together for the attractive nuclear force to take over
Different isotopes of Hydrogen fuel Fusion reactions.
Fusion reactions are most easily fueled by deuterium and
tritium isotopes, or D-T fuel. D-T fusion reactions release over
four times as much energy as uranium fission on a basis of
mass [1]. Deuterium is abundant, occurring naturally in
seawater. Tritium, although not largely naturally occurring,
can be created in a fusion system from lithium, which is
present in large quantities on earth [1]. A blanket containing
lithium which surrounds the reactors core will absorb Neutrons
produced by D-T reactions. This causes the lithium to be
transformed into tritium, which will then act to fuel further
reactions, and helium. Finally, although D-T reactions can
produce a great energy yield fusion has a much lower power
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John Hiller
Benjamin Page
The plasma’s ions and electrons travel along generated
magnetic field lines, but are unable to cross the lines [3].
Magnetic confinement systems are constructed in
toroidal, or doughnut shaped, configuration. Toroidal
configurations have their magnetic fields curved around them
to form closed loops [1]. Also, a superimposed perpendicular
field component, or poloidal field, is required for proper
confinement. The result is a magnetic field with spiral, or
helical, path force lines that confine the plasma.
density than fission. This means that fusion reactors will
require greater sizes and costs than fission reactors.
The use of Tritium as part of the fusion fuel creates a
possible issue associated with nuclear fusion. Tritium is
weakly radioactive and, although it may not represent a great
environmental concern, it can be harmful if brought directly
into contact with the human body [1]. Because of this there is
interest in the use of Deuterium Deuterium, or D-D reactions,
but these would require even greater temperatures and thusly
this form of fuel is often only used in small-scale research
reactors. Future use of D-D reactions means tritium only
represents a temporary problem. However if a large enough
safety concern is created, Tritium could still represent a
possible delay when nuclear fusion finally begins large scale
commercial implementation.
An understanding of the fusion fuel allows for a better
explanation of the sustainability of nuclear fusion. The
components of D-T fuel can be easily found on earth, or
created directly inside of a fusion device. Additionally, if D-D
fuel is ever achieved on a commercial level, fusion reactions
will easily be fueled by an abundant natural resource. The
challenge associated with fusion is the construction of a device
capable continuous facilitation of fusion reactions.
Facilitating Fusion Reactions
For a fusion reaction to successfully occur on earth a
device must be developed which can heat the D-T fuel to
temperatures of the order of 50 million degrees Celsius while
also keeping it under intense pressure. This will allow the fuel
to be kept dense enough and confined long enough for the
hydrogens nuclei to fuse. Fusion research programs work
towards achieving what is known as ignition. Ignition is the
point at which fusion reactions produce net energy and become
self-sustaining, only requiring fresh fuel to be added to
continue it [1].
When ignition is finally achieved, the net energy
produced is about four times greater than that of a fission
reaction. This resulting greater energy yield represents the
potential and importance of developing the future of fusion
technology. Currently one method of facilitating fusion
reactions being studied is magnetic confinement, which makes
use of magnetic fields to contain the plasma fuel.
FIGURE 1 [4]
Simple Diagram of Field Components
Figure 1 offers a simple illustration of the toroidal and
poloidal field components required in the use of magnetic
confinement system. Also included is the resulting helical path
which the plasma fuel will be confined along
The effectiveness of the toroidal shaped reactor is
seen within the helical path the plasma fuel is confined along.
The fuel forms a closed loop which travels throughout the torus
shape, but remains isolated from the reactor walls. One form
of toroidal confinement system which is receiving various
international research is the tokamak.
MAGNETIC CONFINEMENT SYSTEMS
THE TOKAMAK
One method of facilitating fusion reactions which has
received a heavy amount of research and shown potential for
success is using Magnetic Confinement systems. Magnetic
confinement systems make use of strong magnetic fields to
confine D-T plasma under atmospheres of pressure while it is
heated to fusion temperatures [1]. These strong magnetic fields
isolate the plasma from air by confining them in a vacuum
vessel.
The use of magnetic fields to confine plasma are
ideal, because of the plasma’s separated electrons and ions.
Tokamaks are a form of magnetic confinement
system originally developed in 1951 by soviet physicists
Andrei Sakharov and Igor Tamm [1]. Tokamaks lack the
complexity present in the designing and building of the
stellarator, another form of magnetic confinement system [1].
The simpler design has caused tokamaks to become more
favored, leading to abundant international research in the form
of several small-scale tokamaks. Because of their lack of
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John Hiller
Benjamin Page
complexity and the large amount of research invested into
them, tokamaks represent what could soon become an
accessible method of commercial use of nuclear fusion power.
How Tokamaks Work
Within a tokamak an external coil, the central
solenoid, and the plasma fuel itself act as the respective
primary and secondary windings of a transformer [3]. A
current is induced in the plasma by a change of current in the
central solenoid. This current also provides a portion of the
heating required for the plasma to undergo the fusion reaction.
The remaining portion of heating required is supplied through
high-frequency heating, which makes use of electromagnetic
waves of different frequencies. These waves heat the plasma
through resonant absorption, meaning energized neutral
particles penetrate the plasma and transfer their kinetic energy
through collisions with plasma particles [3].
Toroidal and poloidal field coils which surround torus
shaped vacuum vessel which isolates the plasma from air
generate the confining magnetic field [3]. The toroidal coils
are evenly spaced vertically around the vessel and generate the
toroidal field and the poloidal coils, which generate the
perpendicular field component, are placed horizontally around
the toroidal coils [1]. The resulting helical magnetic field
generally has a strength of around 5 tesla, about 100,000 times
as strong as earth’s magnetic field [3].
The inside of the vacuum vessel is lined with lithium
containing blanket modules, which react with the neutrons
resulting from fusion reactions [3]. This in turn produces
additional tritium to fuel further reactions and causes the
neutrons energy to be removed from the vessel and heat a water
circuit which produces steam to power the actual electrical
generators. Further components include a divertor which
removes impurities and Helium resulting from fusion reactors
from the vacuum vessel, and a cryostat which keeps the
superconducting magnets cooled to their operating
temperature of -269 degrees Celsius [3].
FIGURE 2 [4]
Diagram of major tokamak components
The above diagram offers a look at the configurations
of the major tokamak components. Included in the diagram are
the toroidal and poloidal coils, their filed components, and the
resulting helical magnetic field, as well as the external coil and
current induced in the plasma, which correspond to the primary
and secondary transformer circuits.
The tokamak offers a simple design which makes use
of magnetic fields to allow for fusion reactions to be
continuously facilitated. Despite the large scale a commercial
tokamak will require to produce usable energy, the tokamaks
design has continuously been successfully tested by many
international research reactors
EXAMPLES OF TOKAMAKS
Many examples of operational tokamaks, which are
built on a small-scale for research purposes, exist
internationally today. Additionally, there are certain
international efforts being made to develop large-scale
research tokamaks, which will act to imitate the operations of
commercial tokamaks. Both small-scale and large-scale
tokamaks act as valuable resources of information of the
behavior of tokamak confinement systems and the possibilities
of nuclear fusion.
JET the Largest Operating Tokamak
The Joint European Torus, or JET, is an operational
research tokamak used by more than 40 European laboratories,
with 350 scientists and engineers contributing to its operation
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Benjamin Page
[4]. JET originally began operation in 1983. As of July, of
2014 a 283-million-euro contract signed between the European
Commission and the Culham Centre for Fusion Energy, or
CCFE, which secured JET’s operation until 2018.
The original purpose of JET was to study the plasma
behavior in conditions mimicking those of a commercial
fusion reactor. JET is the largest and most powerful tokamak
in the world and the only device capable of making use of the
D-T plasma fuel which is planned for use in commercial
reactors [4]. This is important because, unlike other smaller
research tokamaks which make use of D-D fuel, JET allows
for the closest possible look at the plasma behaviors in future
D-T devices. However, today its purpose is to act as
preparation for the ITER tokamak, another research tokamak
which has not yet been constructed.
Throughout the years of its operation, JET has
received upgrades to make it more like ITER so that it can
better prepare for ITER’s operation. One important upgrade
was to equip JET’s vacuum vessel’s inner wall with the same
materials which will be used in ITER’s construction, beryllium
and tungsten [4]. The new wall, in association with upgraded
heating power, will allow for plasma scenarios closely
resembling those expected in ITER. JET experiments have
already allowed for the decision to be made to have ITER
operate with a fully tungsten divertor, which will reduce
investment costs.
Another important task JET has been set to is ELM
mitigation [4]. ELMs, or Edge Localized Modes, are short
plasma outbursts which cause large heat and particle loads to
be thrust onto the vacuum vessel walls. Within devices as
powerful as ITER, ELMs represent a large risk for the vessel
wall, because of this JET has been tasked with prediction and
mitigation of these events.
Both JETs early and more recent operation purposes
have been very important for the future of nuclear fusion
power. Before JET was tasked with aiding in the preparation
of ITER it offered a valuable resource for understanding the
behavior of the D-T plasma which will be used in large-scale
tokamaks such as ITER. However, after it began to be
upgraded to more resemble ITER, JET became even more
important for the future development of commercial nuclear
fusion. Because ITER will be designed on a much larger,
commercial mimicking scale, having JET offer a possible
preview of how ITER will behave will likely allow for ITERs
operators to be better prepared following ITERs construction.
Additionally, as it continues its operation, JET could continue
to allow for a better understanding of how ITER must be
constructed to be most successful, possibly allowing for
further reduction of investment costs, and what can be
expected in ITERs operation.
ITER, international thermonuclear experimental
reactor, is a research tokamak being developed through a 35nation collaboration. ITER’s purpose is to prove the feasibility
of a tokamak system on a scale large enough for commercial
implementation [5]. The idea for ITER first began in 1985 and,
since then, has received design contributions form thousands
of engineers and scientists. ITER will act to bridge the gap
between preceding small-scale research tokamak devices and
future large-scale commercial tokamak devices.
ITER’s purpose is to achieve net energy production,
maintain fusion for long periods of time, and test the tokamak
designs when producing energy necessary for commercial
production [5]. ITER will have ten times the plasma volume as
JET, the current largest operating tokamak. Because the
amount of fusion energy is a direct result of the amount of
fusion reactions taking place within a fusion reactor, this
increased volume will allow for the large-scale fusion
reactions needed for ITERs goals.
Currently the largest amount of fusion power ever
produced on earth is 16 megawatts [5]. JET achieved this
output in 1997, and required a total power input of 24
megawatts. ITER is designed to produce a record breaking 500
megawatts of fusion and at the input cost of only 50 megawatts
of power. Although ITER will not be designed to capture the
produced energy as electricity it will be the first fusion
experiment to produce a net energy gain. ITER is also expected
to achieve a so called “burning plasma,” in which the heat of a
fusion reaction is confined within the plasma fuel. This will
allow for longer sustained reactions which have never been
achieved.
ITER will also act as an experiment for the
capabilities of in-vessel tritium breeding [5]. This is important
to the future of fusion technology because of the lack of
naturally occurring tritium. Successful tritium breeding will
allow for proof of a continuously sustainable fusion fuel for
future commercial fusion devices.
Finally, ITER will be demonstrative of the general
operation and safety of commercial scale fusion devices. ITER
will also act to demonstrate the negligible environmental
consequences present in large scale fusion fuel synthesis and
fusion reactions.
ITER represents a large step in the direction of
commercial nuclear fusion. Although still experimental, ITER
will produce energy which will dwarf levels present in
previous research tokamaks and, more importantly, this energy
will heavily outweigh the energy input into the device. ITER
is not expected to have its first plasma prepared until
December of 2025, and D-T operations are projected to begin
in 2035 [5].
ITER, JETs Successor
K-DEMO a Step for Tokamaks after ITER
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John Hiller
Benjamin Page
resource in ensuring the safety of operation when faced large
amounts of runaway currents, such as those expected in ITERs
operation. This will likely allow for more reliability in future
tokamak operation.
Following a Korean fusion energy development
promotion law, or FEDPL, enacted in 2007 a design for a
Korean fusion demonstration reactor, or K-DEMO, was
initiated in 2012 [6]. K-DEMOs construction is expected by
2037 with a scale comparable to that of ITER. However, a
special development plan may cause K-DEMO to represent the
commercial next step for Tokamaks.
A special concept which has been proposed as a
possibility for K-DEMO is a development plan over two
phases. Its first phase will resemble purposes like that of
ITER. K-DEMO will design a net energy gain and sustained
in-vessel tritium breeding, as well as acting as a component
test facility. During its second phase, in-vessel components
will be upgraded. This upgrade will allow K-DEMO to show
not only net energy gain, but also net electric generation of
about 500 MW.
Unlike ITER, K-DEMO will work to harness its
energy gain as electric power, rather than just demonstrating
large scale net energy. Therefore K-DEMO can be viewed as
a likely next step for commercial tokamaks.
IMPACTS OF TOKAMAKS
Despite the research heavily invested into the safety
and reliability of tokamaks, possible negative impacts are still
present in future commercial use. One such impact lies in the
use of D-T plasma fuel, specifically in the tritium isotope.
Although tritium does not represent a heavy environmental
concern, it is weakly radioactive. Due to it being an isotope of
hydrogen, it can enter the human body and cause health
concerns [8]. Because an increase in local levels of tritium
have been observed in association with nuclear devices, this
could represent a need for greater safety to be applied to
tokamaks before international commercial implementation.
Another issue associated with the tokamak has to do
with the producing of certain components and the associated
price. A tokamaks major price contributions have to do with
its toroidal and poloidal field coils, and its use of an external
coil. Tokamak filed coils are much larger than those associated
with other magnetic confinement systems, which means when
they must be built directly on the devices construction site, or
shipped in through unconventional means [9]. Also, the
external coil which the tokamak makes use is not present in
other magnetic confinement systems. This means that the
tokamaks encounter an additional price point when compared
to alternative confinement systems to achieve a simpler and
easier to implement design.
K-STAR and ECRH
The Korea superconducting tokamak advanced
research device, or K-STAR, is a small-scale research tokamak
which has acted to investigate the behavior of relativistic
runaway electrons during electron cyclotron resonance
heating, or ECRH [7]. Relativistic runaway electrons
generated within tokamaks can cause severe damage through
collisions with vacuum vessel walls. Because ITER is
expected to experience a large amount of runaway currents
research into runaway electrons is of great importance.
Regularly, the growth rate of runaway electrons can
be diminished by increases in electron density [7]. However,
the density required for a large-scale tokamak such as ITER
may not be accomplishable and, even if it were, it may
adversely affect the vacuum vessel. K-STAR therefore
underwent experimentation of runaway electron growth rate
decrease using ECRH discharges. K-STAR observed the
electron behavior through brief usage of the hard x-ray, or
HXR, monitor system [7]. Through its experimentation, KSTAR found that ECRH discharges could lower runaway
electron discharge rates, but also experienced an unexpected
phenomenon. It was found that under certain plasma
conditions superthermal electron generation occurred during
ECRH. This meant that tokamaks using ECRH must take care
to observe plasma conditions when employing ECRH based
suppression.
K-STARs research is important because of the
service it provided for ITER and future commercial tokamaks.
Large scale tokamaks have been provided with a valuable
THE FUTURE FOR TOKAMAKS AND
NUCLEAR FUSION
Despite the progress being made in the research of
tokamaks and their future implementation, commercial use of
nuclear fusion is still a long way off. Still, many strides toward
commercial tokamaks have been made and through research
projects like ITER the potential for nuclear power can be
presented on an international level. Additionally, although the
implementation and continued operation of tokamaks will
largely be a job for future generations, it can be seen through
projects like K-DEMO that commercial nuclear fusion has not
simply been disregarded as solely a technology reserved for
the future. Through the combined efforts of scientists and
engineers of today and tomorrow, the use of large scale nuclear
fusion can be achieved and implemented on an international
level.
SOURCES
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John Hiller
Benjamin Page
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[8] “Tritium and the Environment” 8.9.2012 Accessed
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[8] R. Siemon, I. Lindemuth, K. Schoenberg. “Why
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y-MTF-Comments.html
ACKNOWLEDGMENTS
The authors would like to acknowledge their fellow
Forbes hall floor 4 residents who, through their concurrent
hard work helped to motivate us in writing our paper.
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